Long term evolution (LTE) may be a standard for wireless data communication technology that delivers a high data rate, low latency and packet optimized radio access technology. LTE, however, supports only a packet switched (PS) service while a universal mobile telecommunications system radio access network (UTRAN) supports both of a circuit switched (CS) service and a packet switched (PS) service. In order to provide a CS service in a LTE network, circuit switched fallback may be employed in the LTE network.
The CS fallback may enable the provisioning of voice and other CS services by reuse of a GSM/EDGE radio access network (GERAN) or a UTRAN CS infrastructure when user equipment is served by a LTE network. GSM may by an abbreviation of a global system for mobile communications and EDGE may be abbreviation of enhanced data rates for GSM evolution. The CS fallback is only available if LTE service coverage is overlapped with service coverage of a GERAN or a UTRAN CS network. For example, in the LTE network, a CS fallback procedure may hand over user equipment from a serving LTE cell to an associated non-LTE cell in order to process a mobile originated (MO) call or a mobile terminated (MT) call while the user equipment is camping on the serving LTE cell. The non-LTE cell may be a GERAN cell or a UTRAN cell. The LTE cell may be mapped to a corresponding non-LTE cell in a one-to-one relation. A CS fallback call may be transferred from the serving LTE cell to the corresponding non-LTE cell in the CS fallback procedure.
In case of a UTRAN including a wideband code division multiple access (WCDMA) network, multiple frequency assignments (FAs) may be used by one UTRAN cell. Frequency assignment (FA) may denote a frequency channel unit that can be allocated to each a cell base station. That is, each cell base station may be allocated with a plurality of carriers to provide a corresponding service to user equipment. Accordingly, multiple logical UTRAN cells may be vertically overlaid on a LTE cell although one physical UTRAN cell is overlaid on a LTE cell. During a CS fallback procedure, one UTRAN cell may be selected as a target cell for handover of user equipment. Since one UTRAN cell may use multiple carriers, a carrier to provide a CS fallback service may be properly selected from the allocated multiple carriers based on radio environments of thereof.